Network Working Group K. Moore
Request for Comments: 2964 University of Tennessee
BCP: 44 N. Freed
Category: Best Current Practice Innosoft
October 2000
Use of HTTP State Management
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
IESG Note
The IESG notes that this mechanism makes use of the .local top-level
domain (TLD) internally when handling host names that don't contain
any dots, and that this mechanism might not work in the expected way
should an actual .local TLD ever be registered.
Abstract
The mechanisms described in "HTTP State Management Mechanism" (RFC-
2965), and its predecessor (RFC-2109), can be used for many different
purposes. However, some current and potential uses of the protocol
are controversial because they have significant user privacy and
security implications. This memo identifies specific uses of
Hypertext Transfer Protocol (HTTP) State Management protocol which
are either (a) not recommended by the IETF, or (b) believed to be
harmful, and discouraged. This memo also details additional privacy
considerations which are not covered by the HTTP State Management
protocol specification.
1. Introduction
The HTTP State Management mechanism is both useful and controversial.
It is useful because numerous applications of HTTP benefit from the
ability to save state between HTTP transactions, without encoding
such state in URLs. It is controversial because the mechanism has
been used to accomplish things for which it was not designed and is
not well-suited. Some of these uses have attracted a great deal of
public criticism because they threaten to violate the privacy of web
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users, specifically by leaking potentially sensitive information to
third parties such as the Web sites a user has visited. There are
also other uses of HTTP State Management which are inappropriate even
though they do not threaten user privacy.
This memo therefore identifies uses of the HTTP State Management
protocol specified in RFC-2965 which are not recommended by the IETF,
or which are believed to be harmful and are therefore discouraged.
This document occasionally uses terms that appear in capital letters.
When the terms "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
appear capitalized, they are being used to indicate particular
requirements of this specification. A discussion of the meanings of
the terms "MUST", "SHOULD", and "MAY" appears in [RFC-1123]; the
terms "MUST NOT" and "SHOULD NOT" are logical extensions of this
usage.
2. Uses of HTTP State Management
The purpose of HTTP State Management is to allow an HTTP-based
service to create stateful "sessions" which persist across multiple
HTTP transactions. A single session may involve transactions with
multiple server hosts. Multiple client hosts may also be involved in
a single session when the session data for a particular user is
shared between client hosts (e.g., via a networked file system). In
other words, the "session" retains state between a "user" and a
"service", not between particular hosts.
It's important to realize that similar capabilities may also be
achieved using the "bare" HTTP protocol, and/or dynamically-generated
HTML, without the State Management extensions. For example, state
information can be transmitted from the service to the user by
embedding a session identifier in one or more URLs which appear in
HTTP redirects, or dynamically generated HTML; and the state
information may be returned from the user to the service when such
URLs appear in a GET or POST request. HTML forms can also be used to
pass state information from the service to the user and back, without
the user being aware of this happening.
However, the HTTP State Management facility does provide an increase
in functionality over ordinary HTTP and HTML. In practice, this
additional functionality includes:
(1) The ability to exchange URLs between users, of resources
accessed during stateful sessions, without leaking the state
information associated with those sessions. (e.g. "Here's the
URL for the FooCorp web catalog entry for those sandals that
you wanted.")
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(2) The ability to maintain session state without "cache-busting".
That is, separating the session state from the URL allows a web
cache to maintain only a single copy of the named resource. If
the state is maintained in session-specific URLs, the cache
would likely have to maintain several identical copies of the
resource.
(3) The ability to implement sessions with minimal server
configuration and minimal protocol overhead, as compared to
other techniques of maintaining session state.
(4) The ability to associate the user with session state whenever a
user accesses the service, regardless of whether the user
enters through a particular "home page" or "portal".
(5) The ability to save session information in stable storage, so
that a "session" can be maintained across client invocations,
system reboots, and client or system crashes.
2.1. Recommended Uses
Use of HTTP State Management is appropriate whenever it is desirable
to maintain state between a user and a service across multiple HTTP
transactions, provided that:
(1) the user is aware that session state is being maintained and
consents to it,
(2) the user has the ability to delete the state associated with
such a session at any time,
(3) the information obtained through the ability to track the
user's usage of the service is not disclosed to other parties
without the user's explicit consent, and
(4) session information itself cannot contain sensitive information
and cannot be used to obtain sensitive information that is not
otherwise available to an eavesdropper.
This last point is important because cookies are usually sent in the
clear and hence are readily available to eavesdroppers.
An example of such a recommended use would be a "shopping cart",
where the existence of the shopping cart is explicitly made known to
the user, the user can explicitly "empty" his or her shopping cart
(either by requesting that it be emptied or by purchasing those
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items) and thus cause the shared state to be discarded, and the
service asserts that it will not disclose the user's shopping or
browsing habits to third parties without the user's consent.
Note that the HTTP State Management protocol effectively allows a
service provider to refuse to provide a service, or provide a reduced
level of service, if the user or a user's client fails to honor a
request to maintain session state. Absent legal prohibition to the
contrary, the server MAY refuse to provide the service, or provide a
reduced level of service, under these conditions. As a purely
practical consideration, services designed to utilize HTTP State
Management may be unable to function properly if the client does not
provide it. Such servers SHOULD gracefully handle such conditions
and explain to the user why the full level of service is not
available.
2.2. Problematic Uses
The following uses of HTTP State Management are deemed inappropriate
and contrary to this specification:
2.2.1. Leakage of Information to Third Parties
HTTP State Management MUST NOT be used to leak information about the
user or the user's browsing habits to other parties besides the user
or service, without the user's explicit consent. Such usage is
prohibited even if the user's name or other externally-assigned
identifier are not exposed to other parties, because the state
management mechanism itself provides an identifier which can be used
to compile information about the user.
Because such practices encourage users to defeat HTTP State
Management mechanisms, they tend to reduce the effectiveness of HTTP
State Management, and are therefore considered detrimental to the
operation of the web.
2.2.2. Use as an Authentication Mechanism
It is generally inappropriate to use the HTTP State Management
protocol as an authentication mechanism. HTTP State Management is
not designed with such use in mind, and safeguards for protection of
authentication credentials are lacking in both the protocol
specification and in widely deployed HTTP clients and servers. Most
HTTP sessions are not encrypted and "cookies" may therefore be
exposed to passive eavesdroppers. Furthermore, HTTP clients and
servers typically store "cookies" in cleartext with little or no
protection against exposure. HTTP State Management therefore SHOULD
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RFC 2964 Use of HTTP State Management October 2000
NOT be used as an authentication mechanism to protect information
from being exposed to unauthorized parties, even if the HTTP sessions
are encrypted.
The prohibition against using HTTP State Management for
authentication includes both its use to protect information which is
provided by the service, and its use to protect potentially sensitive
information about the user which is entrusted to the service's care.
For example, it would be inappropriate to expose a user's name,
address, telephone number, or billing information to a client that
merely presented a cookie which had been previously associated with
the user.
Similarly, HTTP State Management SHOULD NOT be used to authenticate
user requests if unauthorized requests might have undesirable side-
effects for the user, unless the user is aware of the potential for
such side-effects and explicitly consents to such use. For example,
a service which allowed a user to order merchandise with a single
"click", based entirely on the user's stored "cookies", could
inconvenience the user by requiring her to dispute charges to her
credit card, and/or return the unwanted merchandise, in the event
that the cookies were exposed to third parties.
Some uses of HTTP State Management to identify users may be
relatively harmless, for example, if the only information which can
be thus exposed belongs to the service, and the service will suffer
little harm from the exposure of such information.
3. User Interface Considerations for HTTP State Management
HTTP State Management has been very controversial because of its
potential to expose information about a user's browsing habits to
third parties, without the knowledge or consent of the user. While
such exposure is possible, this is less a flaw in the protocol itself
than a failure of HTTP client implementations (and of some providers
of HTTP-based services) to protect users' interests.
As implied above, there are other ways to maintain session state than
using HTTP State Management, and therefore other ways in which users'
browsing habits can be tracked. Indeed, it is difficult to imagine
how the HTTP protocol or an HTTP client could actually prevent a
service from disclosing a user's "click trail" to other parties if
the service chose to do so. Protection of such information from
inappropriate exposure must therefore be the responsibility of the
service. HTTP client implementations inherently cannot provide such
protection, though they can implement countermeasures which make it
more difficult for HTTP State Management to be used as the mechanism
by which such information is exposed.
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RFC 2964 Use of HTTP State Management October 2000
It is arguable that HTTP clients should provide more protection in
general against inappropriate exposure of tracking information,
regardless of whether the exposure were facilitated by use of HTTP
State Management or by some other means. However, issues related to
other mechanisms are beyond the scope of this memo.
3.1. Capabilities Required of an HTTP Client
A user's willingness to consent to use of HTTP State Management is
likely to vary from one service to another, according to whether the
user trusts the service to use the information appropriately and to
limit its exposure to other parties. The user therefore SHOULD be
able to control whether his client supports a service's request to
use HTTP State Management, on a per-service basis. In particular:
(1) Clients MUST NOT respond to HTTP State Management requests
unless explicitly enabled by the user.
(2) Clients SHOULD provide an effective interface which allows
users to review, and approve or refuse, any particular requests
from a server to maintain state information, before the client
provides any state information to the server.
(3) Clients SHOULD provide an effective interface which allows
users to instruct their clients to ignore all requests from a
particular service to maintain state information, on a per-
service basis, immediately in response to any particular
request from a server, before the client provides any state
information to the server.
(4) Clients SHOULD provide an effective interface which allows a
user to disable future transmission of any state information to
a service, and/or discard any saved state information for that
service, even though the user has previously approved a
service's request to maintain state information.
(5) Clients SHOULD provide an effective interface which allows a
user to terminate a previous request not to retain state
management information for a given service.
3.2. Limitations of the domain-match algorithm
The domain-match algorithm in RFC-2965 section 2 is intended as a
heuristic to allow a client to "guess" whether or not two domains are
part of the same service. There are few rules about how domain names
can be used, and the structure of domain names and how they are
delegated varies from one top-level domain to another (i.e. the
client cannot tell which part of the domain was assigned to the
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service). Therefore NO string comparison algorithm (including the
domain-match algorithm) can be relied on to distinguish a domain that
belongs to a particular service, from a domain that belongs to
another party.
As stated above, each service is ultimately responsible for ensuring
that user information is not inappropriately leaked to third parties.
Leaking information to third parties via State Management by careful
selection of domain names, or by assigning domain names to hosts
maintained by third parties, is at least as inappropriate as leaking
the same information by other means.
4. Security Considerations
This entire memo is about security considerations.
5. Authors' Addresses
Keith Moore
University of Tennessee Computer Science Department
1122 Volunteer Blvd, Suite 203
Knoxville TN, 37996-3450
EMail: moore@cs.utk.edu
Ned Freed
Innosoft International, Inc.
1050 Lakes Drive
West Covina, CA 81790
EMail: ned.freed@innosoft.com
6. References
[RFC 1123] Braden, R., "Requirements for Internet Hosts --
Application and Support", STD 3, RFC 1123, October 1989.
[RFC 2965] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2965, October 2000.
[RFC 2109] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2109, February 1997.
Moore & Freed Best Current Practice [Page 7]RFC 2964 Use of HTTP State Management October 2000
7. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Moore & Freed Best Current Practice [Page 8]
